Kaonicping updated Sandbound with a new update entry: 1.3.1: a few small changes Read the rest of this update entry...
I think you should still do it though. Make the boss dungeons and bosses as seperate mods on their own and let people download them, you can release it as a boss modpack so people can fight bosses without needing to go through the missions to do so. As for Sandbox, it's more or less cancelled since the author said he's losing interest in it. But it's up to you though.